1. Field of the Invention
The present invention relates to atmospheric dispersion monitoring and, more particularly, to a system for tracing atmospheric dispersion by means of release, collection and analysis of one or more tracer gases, particularly sulfur hexafluoride.
2. Description of the Prior Art
The spread and dilution of atmospheric contaminants from real sources over distances and in time is of great practical concern. Both small and large scale information is necessary for design of ventilation systems in buildings, location of residential communities with respect to dispersion of pollutants from an urban highway, pollutant flow within the wake downwind of a building and the large scale transport and dispersion within a single basin and extending into adjacent basins.
In general, models for predicting atmospheric dispersion of pollutants over long distances can be divided into two classes: those which assume that concentrations vary in a Gaussian manner, commonly known as Gaussian plume models, and those which use the concept of an eddy diffusivity, commonly known as K-theory. However, in many cases the air flow is so complicated that theoretical solutions of the governing fluid mechanics equations prove intractable. Experimental tracer studies become a necessity in these cases providing basic concentration data for the formulation of equations describing the dispersion process. Before any theory of atmosphere dispersion can be considered valid, it must be compared with experimental observations of a tracer substance which moves and disperses within the main air flow.
Many different types of tracers have been used to experimentally study atmospheric dispersion. Three general categories of tracers have been considered: optical outline methods, trajectories of individual markers, and chemical tracer substances which disperse along the flow. Oil-fog, dense black smoke has been commonly used in optical outline tracer tests. The problem with optical outline methods is that they are generally limited to close distances from the source and dilution factors can be only roughly estimated from photographs.
The trajectories of balloons equipped with radar transponders can be individually followed for fairly long distances. However, balloons can only be used for wind speed and direction data; information on the dilution of pollutant concentrations is not possible.
For accurate dilution information, chemical tracer substances which disperse along the main flow must be used. Chemical tracers can be categorized as particulates and gases. The most widely used tracer substance has been a fluorescent particulate mixture of zinc and cadmium sulfide, sometimes referred to as simply FP tracer. After release of the tracer, the analysis required drawing an air sample through filter paper and then counting the number of fluorescent particles trapped on the paper under ultraviolet light. This tracer technique, though widely used, will decline in the future because the tracer has been classified as toxic which limits its use in heavily populated areas and another problem is that at distances of 60-70 km, typically 50-80% of the particles had settled out and been lost.
Gaseous tracers have a distinct advantage over particulate tracers since they do not settle out due to gravitational forces. Several radioactive gaseous tracers have been utilized and have been traced for distances as great as 160 miles. With caution, these tracers may be suitable for testing small-scale ventilation problems, but are not appropriate for use over populated areas. Even when used in dilute amounts for testing the ventilation in the room, the experimentor must monitor concentrations remotely from another room.
Sulfur dioxide has been used to monitor dispersion downwind of a power plant. However, sensitivity measurement is a problem as is interference from existing and interfering background SO.sub.2 concentrations. A recent study suggested measuring sulfur-32 to sulfur-34 isotope ratios with a mass spectrometer as a method for tracing stack emissions. In small enclosures, initial high levels of the order of 0.5% of various tracer gases such as helium, nitrous oxide or volatile organic solvents can be introduced to overcome sensitivity problems in the analysis.
Recent advances in analytical chemistry have made possible extremely sensitive measurements of certain gaseous compounds. The principle of detection is based on the fact that these gases have a very high affinity for capturing electrons, and if these gases are introduced into a small electric current, the measured current will increase. The electron capture detector is a substance specific device. It is extremely sensitive to certain molecular species which react with free electrons to form stable negative ions such as molecules containing electron absorbing groups such as halogens, carbonyl, nitro or certain condensed ring aromatics. However, the EC detector has very low sensitivity for hydrocarbons other than fused ring aromatics and importantly it can reliably detect halogenated hydrocarbons in quantities as low as 10.sup..sup.-10 to 10.sup..sup.-12 and, under optimized conditions, 10.sup..sup.-14 concentrations have been detected.
Studies of the sensitivity of electron capture detectors for a large number of halogenated compounds showed that sulfur hexafluoride, an inert non-toxic gas, had one of the highest responses, and can be detected in quantities of as low as 10.sup..sup.-13 parts SF.sub.6 per part of air. SF.sub.6 is chemically inert at normal temperatures, is stable to ultraviolet light and is extremely insoluble in water. When animals were exposed to atmospheres consisting of 80% SF.sub.6 and 20% O.sub.2 for periods of 16-24 hours, no indications of intoxication, irritation or any other symptoms were observed during the exposures or any time thereafter. Human subjects were recently exposed to a breathing mixture of 90% SF.sub.6 and 10% O.sub.2 under pressure of 2 atmospheres. In a similar study, dogs were exposed to a breathing mixture of 95% SF.sub.6 and 5% O.sub.2 at a pressure of 4 atmospheres. No mention of any abnormal or toxic effects was made in either of the latter two studies.
Thus, since it is gaseous, physiologically inert, chemically inert, and easily detectable in extremely low concentrations, sulfur hexafluoride is an excellent atmospheric tracer. Long range studies have shown that SF.sub.6 can be detected with ease beyond 70 miles away from a continuous point source. The extreme sensitivity of this tracer which can readily be measured at one part in 10.sup.12 is necessary for large-scale tracer tests. As an example of the sensitivity, if only 100 lbs of SF.sub.6 were allowed to evenly disperse up to a height of 1/4 mile, the tracer could be detected over an area of 8,000 square miles, which is approximately the area of the state of Massachusetts.
Tracer studies require the collection of air samples to be returned to the laboratory for analysis or for analysis at the field site using portable equipment. To obtain ambient air samples, sometimes called grab samples, previous studies have utilized evacuated stainless steel cylinders, glass containers, plastic bags and plastic squeeze bottles. Steel cylinders are extremely effective for samples since there is low absorption, and the time for sampling can be easily adjusted. However, stainless steel cylinders are expensive and quite bulky. Glass containers are not suitable since the surface charge on the glass surface adsorbs electron absorbing compounds such as SF.sub.6. Plastic bags are relatively inexpensive; however, they are also bulky and require a pump which can contaminate an air sample. Plastic squeeze bottles are inexpensive, take up little space and can be easily cleaned for reuse. The sampling procedure requires squeezing the bottle hard 10 successive times, one squeeze a second. Though this procedure shows good reproducibility, it requires the presence of an operator to open, collect and close the sample bottle and the bottle is not readily adaptable for automated sampling or analysis.
The gas chromatograph-electron capture analysis devices that are commercially available are not specifically designed for tracer gas analysis, are unnecessarily complex, bulky, and expensive and must be modified in order to be suitable for use in such studies.